1. Field of the Invention
The present invention relates to a photosensitive polyimide for use as a liquid photo resist or dry film resist, or for use in a solder resist, coverlay film, or printed wiring board.
2. Description of the Prior Art
These days, electronic products are made light and small. Therefore, various electronic parts have been miniaturized. Since flexible printed wiring boards are excellent in terms of flexibility and weight and resistant to high temperatures, the demand for them has increased. For instance, currently popular electronic products such as mobile phone, LCD, PLED, and OLED all utilize flexible printed wiring boards. Compared with conventional silicon or glass substrates, flexible printed wiring boards, also called soft boards, have better flexibility. Normally, flexible printed wiring boards consist of an insulated substrate, a binder, and a copper conductor. Moreover, to prevent the copper circuits on flexible printed wiring boards from being oxidized or influenced by the environmental humidity, a coverlay film, which is normally composed of an insulated substrate and a binder, is usually applied onto the boards.
Coverlay film can be classified into three types according to the material: photosensitive coverlay, non-photosensitive coverlay, and thermal plastic coverlay. Photosensitive coverlay can be polyimide based (PI based coverlays) or non-polyimide based (non-PI based coverlays), while utilization of the non-polyimide based coverlay is limited because it has lower heat resistance and a higher coefficient of thermal expansion. As to non-photosensitive coverlay, its manufacture is more complicated than that of photosensitive coverlay, and it is not as useful as photosensitive coverlay. As to thermal plastic coverlay, it needs post processing for forming holes and is not as convenient to use as photosensitive coverlay.
Conventional photosensitive polyimides are formed from the corresponding polyamic acid or polyamic ester precursors. However, since the formation requires high-temperature hard baking (i.e., baking at a temperature more than 300° C.), the copper circuits are liable to be oxidized and the electrical properties and reliability of the products are adversely affected. JP Patent Publication No. 2003-345007 discloses a photosensitive polyimide that can be produced by a process without high-temperature hard baking, the process comprising forming an ionic bonding between an acrylate monomer containing a tertiary amino and a soluble polyimide containing —COOH radicals in the main chain so as to form a negative type photosensitive polyimide. However, the polyimides of this JP case are not suitable for the applications in which thicker polyimide films, coverlay films, or soft boards are necessary.